Inhalers for delivering medicament to a patient by inhalation are known. Such devices include metered dose inhalers and dry-powder inhalers.
Metered-dose inhalers typically comprise a vessel, said vessel containing at least a pressurised propellant and a medicament, and a metering valve and some form of actuator for dispensing doses of the liquid medicament as an aerosol. In contrast, dry-powder inhalers comprise a supply of dry-powder medicament together with means for aerosoling and deagglomerating the medicament into a form suitable for delivery to a patient.
Dry-powder inhalers are generally considered to be easier to use than metered-dose inhalers because of difficulties in coordinating actuation of the metering valve of the metered-dose inhaler with a patent's inspiration. Moreover, since the medicament is aerosolised and delivered using the patient's inspirational airflow, dry-powder inhalers do not require a propellant.
It has been recognised that, in order to aerosolise and deliver a consistent dose of dry-powder medicament in an effective manner, the moisture content of the medicament must be carefully controlled. In dry-powder inhalers this can be conveniently achieved by storing doses of the medicament in the blisters of a sealed blister pack. In use of such devices, a dose of the medicament is typically liberated by piercing, bursting or peeling off a portion of a cover sheet of the blister pack immediately before the dose is to be delivered to the patient for inhalation.
A known dry-powder inhaler of this type, in which doses of the medicament are stored in a blister pack, is disclosed in U.S. Pat. No. 5,590,645. The blister pack of this inhaler is formed as a flexible elongate strip, and comprises a base sheet having a plurality of recesses spaced along its length and a cover sheet hermetically but peelably sealed to the base sheet to define a plurality of blister pockets each containing an identical dose of the medicament.
The inhaler of U.S. Pat. No. 5,590,645 comprises a medicament dispensing mechanism which is arranged to peel a portion of the cover sheet away from the base sheet of the blister pack to thereby open a blister pocket each time the inhaler is actuated. The mechanism includes a blister strip indexing means for moving the opened pocket into registration with medicament holes formed in a manifold of the inhaler, the manifold being in fluid communication with a mouthpiece through which the patient can inhale. An unused portion of the blister strip and used portions of the base sheet and cover sheet are stored within the body of the inhaler.
In use of the above-described inhaler, the patient exposes the mouthpiece by rotating a cover relative to the body of the inhaler, and operates a trigger to actuate the medicament dispensing mechanism. The patient then inhales through the mouthpiece to draw air through the manifold from within the body of the inhaler. The airflow passing through the mouthpiece and the manifold entrains the dry-powder medicament contained within the opened blister pocket, which medicament is delivered to the patient in aerosolised form. The trigger and the cover of the inhaler are then returned to their original positions to reset the medicament dispensing mechanism in preparation for a subsequent use.
Although dry-powder inhalers of the type disclosed in U.S. Pat. No. 5,590,645 exhibit improved consistency in the size of the doses which are delivered to the patient, there remains a need for inhalers having improved dosing consistency. There is a particular need for inhalers in which the risk of over-dosing can be minimised.